That won't increase the slope. Only designing a higher order crossover will increase the slope. As I said, often the wiring indications for second-order are counter-intuitive and sometimes as a result get miswired.
I just found my sheets from years ago when I made the modifications. According to my information the box volume for the 15" is somewhere between 3.2 and 3.5 cubic feet. I estimated the tuning before modifications at 63 Hz.
Depending on driver chosen that would work well in that volume, one might expect 42 to 55 Hz tuning, and I bet the original driver would sim out better lower too - and would say at 65 it is boomy indeed by my present-day tonal stance ; }
FWIW, I think the 8" drivers sound pretty good run full range - better than the BW. They can't take much power that way however....
That's often the tradeoff.
